1. Field of the Invention
This invention relates to a polymer, particularly an acrylamide polymer useful for applications such as an additive for paper making.
2. Description of the Related Art
Polymers including acrylamide polymers have been used in a variety of applications; specifically paper strength agents, surface coatings, drainage aids, retention aids, coagulants, water retainers, dispersants, concrete admixtures, enhanced oil recovery agents. In particular, it has been attempted that, for example, when used as a paper strength agent or a surface coating, an acrylamide polymer is combined with a cross-linking agent to have a partially cross-linked and branched structure, by which the polymer has an increased molecular weight while keeping the viscosity of its solution from being increased, for improving the polymer performance.
For example, there has been proposed a technique using a cross-linking compound for increasing molecular weight and lowering viscosity. Specific compounds used include cross-linking vinyl monomers such as difunctional vinyl monomers including methylene bis(meth)acrylamide, ethyleneglycol di(meth)acrylate, diallyl (meth)acrylamide and divinyl adipate; trifunctional monomers including 1,3,5-triacryloylhexahydro-S-triazine; and tetrafunctional monomers including N,N,N',N'-tetraallyl-1,4-diaminobutane, as well as silicone compounds including vinyltriethoxysilane. A molecular-weight increasing technique based on a cross-linking reaction depending only on such a cross-linking compound, has many problems such as higher nonuniformity of a branched and cross-linked structure; in particular, when using a multifunctional vinyl monomer, the solution may become more viscous with the elapse of time. Thus, such a polymer is not satisfactory as a paper strength agent.
Furthermore, it has been attempted that in addition to these cross-linking compounds, a particular compound is combined for providing an acrylamide polymer with a branched and cross-linked structure. Known combinations include those of a cross-linking compound and a primary or a hydroxylated monomer, a cross-linking compound and a particular hydrophobic monomer, and a cross-linking compound and an itaconic acid derivative. It has been implicated that a polymer from any of these combinations may have an increased molecular weight with a lower viscosity. Such polymers are, however, not satisfactory as a paper strength agent in view of their structures. In fact, they have not provided satisfactory results. Besides the previous improvement, a more improved paper strength agent has been desired.
There have been needs for a paper power agent with a more improved performance because paper-making conditions have been increasingly harsh in a paper making process, such as an increase in a wastepaper rate, an increase of pH value in sheet forming, an increase in a rate of a closed white water due to, e.g., effluent regulation, speedup of a paper machine. Overcoming the problem of reduction in a durability of paper is particularly important. Depending on the application, reduction in a Z-axis directional strength as a component of a durability of a paper may be often a significant problem. Paper power agents for improving a Z-axis directional strength have been disclosed. It is known that a cationic acrylamide polymer obtained by Hofmann degradation of an acrylamide polymer is highly effective. However, higher level of Z-axis directional strength is desired.
Meanwhile, many of surface coatings mainly comprise an anionic polymer. Attempts have been previously made, for example, for increasing a molecular weight by incorporating a cross-linked structure and for saving a transportation cost by concentrating of a coating. However, advanced control techniques of polymer structure have not been introduced.